1. Field of the Invention
The present invention relates generally to a force sensor and, more particularly, to a force sensor that employs a pressure transducer in association with a moveable shaft for transmitting a force to a diaphragm of the pressure sensor.
2. Description of the Prior Art
Many different types of force sensing apparatus are well known to those skilled in the art. In addition, the use of semiconductor components in cooperation with levers is known to the skilled artisan in the field of force sensing. For example, U.S. Pat. No. 3,993,150, which issued to Brosh et al on Nov. 23, 1976, discloses an economical weighing apparatus that employs a cantilever beam structure. The scale utilizes a cantilever beam which has a transducer arrangement positioned on it. The beam is coupled to a platform by means of a rod inserted into an aperture in a linear bearing. It therefore provides a force only in the axial direction and enables the weighing of an object that is positioned at any location on a platform. U.S. Pat. No. 3,790,870, which issued to Mitchell on Feb. 5, 1974, describes a thin oxide force sensitive switch. The bistable solid state switch is provided by copper/cuprous oxide rectifying or blocking junction devices. A contacting thin cuprous oxide layer and an elemental form of a copper substrate have an interface between them. Devices of this type are arranged to avalanche from a blocking state to an essentially conducting state on the application of force when under a related voltage and then avalanche back to a blocking state on the removal of at least a portion of that force. The transition between the states is sharp and predictable, but free of arcing effects. Force switchable diodes of this type provide an inverse relationship between switching force and voltage and a positive relationship between switching force and thickness of the cuprous oxide layer.
Those skilled in the art of electronic circuit design and electronic packaging are familiar with connectors that comprise elastomeric material and a plurality of anisotropic conductors disposed within the elastomeric material to provide selective conductivity only in a preferred direction through the conductor. One supplier of this type of conductor, Fugipoly Incorporated, provides a high accuracy rubber connector referred to as the W series with anisotropic electrical conduction properties. The connectors comprise three hundred to two thousand fine metal wires per cubic centimeter which are embedded in the thickness direction of the transparent silicon rubber sheet. The connectors are inserted between opposite electrodes and provide electrical communication therebetween. The fine metal wires are gold plated to insure that requirements relating to low resistance and the ability to withstand relatively high electrical currents can be met. Fugipoly Incorporated also provides a low resistance elastomeric connector referred to as a ZEBRA which is constructed of alternating parallel layers of electrically conductive and nonconductive silicon elastomer. The conductive layer is filled with silver metal particles. The alternating layers provide electrical connection when placed between two aligned conducting surfaces.
An article titled "Guidelines for Designing Elastomeric Connectors into the System" was written by Leonard Buchoff in the August 1987 edition of Connection Technology. The article describes applications in which elastomeric elements are used in electronic circuit packaging applications where anisotropic conduction is required. In the Dec. 7, 1989 edition of EDN Magazine, Arthur Mosley wrote an article entitled "Elastomeric Connectors/Meet SMT Requirements" which describes the use of the silicone rubber selectively conductive components in various application. It also describes the electrical and temperature characteristics of silicone rubber and its tolerance to ultraviolet radiation, oxygen, ozone and humidity. These articles illustrate the wide knowledge of elastomeric conductors throughout the industry. The information contained in the articles and in the patents described above is expressly incorporated by reference in this application.
Certain applications of force sensors require the sensor to be extremely small, but also to be able to provide an analog signal that is directly proportional to the magnitude of a force being measured. As one particular example, a tactile response pen for use in conjunction with a computer terminal can provide information to the computer describing the magnitude of force that an operator is exerting on the pen against a computer data entry pad. This type of information can be combined with external information relating to the location of the pen contact with the pad to permit the computer to use the magnitude of force as an additional input variable. As one example of a use of this information, the total force can be correlated to the position of the pen to provide thicker or thinner lines resulting from movement of the pen, such as in the deriving of a signature.
Devices such as the application described immediately above require the force sensor to be extremely small while maintaining its accuracy capabilities. In addition, the magnitude of travel of the force sensing component in contact with an external component is very small. It would therefore be significantly beneficial if a force sensor can be made which is small yet accurate and durable.